Pulse signalling system



4, 1947. E ||l B, BARTELINK 2,416,895

PULSE SIGNALLING SYSTEM Filed Oct. 2, 1942 2 fp afp LIM/TER .SYSTEM Everhard HB. Bartelrwk,

Inventor:

by )VME/M His Attorney.

Patented Mar. 4, 1947 PULSE SIGNALLING SYSTEM Everhard H. B. Bartelink, Westl Milton, N. VY., assignor to General Electric Company, a corporation of New York Application October 2, 1942, Serial N0. 460,487

8 Claims.

My invention relates to pulse signalling systems and Aparticularly to such systems used either as radio echo apparatus or point-to-point communication systems. It is an object of my invention to provide means for reducing the noise in such a system.

It is well known that the non-sinusoidal periodic pulses used in a signalling system may be resolved, by Fourier series analysis, into a fundamental sine wave, having a. frequency equal to the rate at which the pulses recur, plus an almost innfinite number of sine Waves whose frequencies are different harmonics of the fundamental. Itis also known that the noise which occurs in radio receivers is irregular in character and covers the frequency band continuously, the noise energy received being proportional to the total band width of the receiver. It is an object of myinvention, therefore, to reduce the noise in the output of the receiver of a pulsing system by providing narrow band response channels corresponding to the discrete harmonic frequencies of the pulse supplied by the transmitter.

In pulse signalling systems used for measuring the distance between a receiver and a reflecting object it is known that the phase relation between direct and reected waves is an indication of the redaction time and may readily be converted into a measurement of the distance between the receiver and the reflecting body. It is another object of my invention to provide a noise reducing circuitfor a pulse signalling system in which the number of band pass channels is reduced to one, `and thus a single harmonic of the reflected wave is selected and its phase compared with the transmitted pulse.

In attempts to utilize the phase difference between two waves as a distance measuring means, since the direct signal, especially when the transmitting apparatus is relatively close to the receiver, is much more intense than the reflected signal, even after limiting of the former, the phase variation between the two signals is difficult to detect. This difficulty may be circumvented by supplying to the receiving apparatus a blanking pulse during the transmitting period. Accordingly, it is a further object of my invention to provide a translating circuit for a pulse signalling system in which a blanking pulse is supplied to the receiver during transmitting periods and the phase interval between the transmission of a signal andthe reception of a reflected signal is measured'. `Another object of my invention is to provide, in afjphase measuring system of the above type, a

(Cl. Z50- 1.66)

2 blanking pulse having a frequency which is a multiple of the transmitter keying pulse frequency.

An additional object of my invention is to provide improved protection for the receiver of a pulse signalling system during a transmitting period by supplying a blanking pulse to desensitize the receiver during such periods.

'I'he features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however,

vtogether with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 shows schematically one form of my noise reduction system for the receiver of a pulse signalling system; Fig. 2 is a block diagram of a system embodying the receiver of Fig. 1; and Fig. 3

is a group of curves illustrating certain operating characteristics of my noise reduction system.

In the radio vreceiving system illustrated in Fig. 1, an antenna I0 is provided for collecting radio frequency waves and supplying them to amplifier II. The specific Adetails of the antenna I0 and the amplifier II form no part of my invention and, therefore, need not be considered. Preferably, the amplier Il'comprises several stages of amplification having a wide band pass characteristic.

The output ofthe amplier Il is coupled by means of transformer I2 to a detector I3, which may be of any suitable type. In Fig. 1 detector I3 has been shown as a diode having its anode I4 connected to one terminal of the tuned circuit comprising the secondary of the transformer I2 and capacitor I5 and its cathode connected to the opposite terminal of this tuned circuit through resistances I 6 and I'I for unidirectional pulse currents and through'capacitor I 8 for radio frequency current. The voltage drop developed across resistor IB may be applied through the lter comprising resistor 20 and condenser 2l, connected to ground, by means of conductor 22 to the amplifier I I for the `usual automatic volume control purpose. 1

The output currents of detector I3 flowing through resistor I6 comprise both signal currents and noise currents. In a pulsing system the signal currents have a regular recurrence, for example lIlOO pulses per second, whereas the noise currents cover the frequency band continuously from very low to very high frequencies. vIn ac- ,cordance with my invention this property of regu- Vlar recurrence of these signal currents is used to 3 discriminate the signal from noise present in the circuit.

Assume that the wave received by the antenna l is of the wave form illustrated by the wave A of Fig. 1. Individual pulses contained in this wave are all substantially rectangular in shape. It is Well known that any non-sinusoidal periodic wave may be resolved into a Fourier series of component waves, each of simple sinusoidal wave form. It is also a demonstrable fact that a periodic wave, composed of identical rectangular pulses recurring at regular intervals, is analyzable into a fundamental sine Wave having a frequency equal to the rate at which the identical pulses recur, plus an almost infinite number of sine waves Whose frequencies are dilerent harmonics of the fundamental. Thus, the frequency of recurrence of the identical pulses determines the lowest frequency present in the sine waves of the series. The amplitude and duration of the individual pulses determine the magnitudes o'f the ccmporlent waves and the particular harmonics present. Distortion of the rectangular wave form does ,not eliminate the fundamental, though the magitudes of the various component waves and the particular harmonics present are altered. These Asine waves of the Fourier series are not merely 'abstract rnathem'atical quantities. They are 'pnysieei realizable electrical waves which Amay easily be derived 'from the lnon-sinusoidal rectan- "gkular, `waves by suitable circuits. Accordingly the 'signal `A can be reproduced without distortion `whene'ver a series of narrow band pass Vfilters is provided, one for each harmonic component.

For the purpose 'discussed above, the 'lters 24, i

2li,4` 26, and 2l are coupled tothe output circuit efuetector I3, filters 24 `and '25 being designed to 'transmita `si`ngle,"and iilters 26 and 21 several, y'of the harmonic components ofthe input wave A. In orderthat one filter does not act as a 'shortlircuit for the frequency which a different Qlte'r 'ls'de's'igned to pass, fit is desirable'that these individual'ltefr's be coupled to the output of detetor 'I3 by some means having a high irnv'pedance, suchas the bulertubes 3U. Fora similarpurposethebuertubes 3fl arefprovided across theuoutpiit of the individuel filter circuits.

The i'llter eirc'uit 24, comprising inductance 32 and capacitor 33, is tuned to resonance at the fundamental frequency of the input wave A, that isthe frequency of occurrence of the pulse of the wave A. As the repetitionratey of this pulse 'usually is derived from a crystal oscillator or i'othergenerator having a very high frequency stability, 'byjm'aking the 5filter v2B a very high quaflity'cireuit, the `pass band of this filter may "be made very narrow. Under such conditions, eurrents fof tlie fundamental frequency only are "supplied `to nthe control grid of the upper buffer Ltube '3`I by `the ltercircuit 24, currents 'of all tlier-frequencie's `being by-passed by this lter circuit.

In a inannersimilar to :that discussed 'with reference to lter circuit 24, lter circuit-2 may be selected to pass the second harmonic component of the ywave A. Similarly, any Vother `nurnber Yof filters may be provided to pass the higher harmonic components of the wave A, the number selected being determined by the delity of reproduction of the wave A desired inthe output Iof Vthe fl'llter circuits.

w 1 timately, the induotances and capacitors of thelter lcircuits may be replaced by a multiple resonint lter, Vsuch as the qa'rt'er `wave eoai'iial 'transmission "lin'e 26 comprising the` "outer 'tube 34 and the inner conductor 35. This type of filter gives maximum transmission at the higher odd numbered harmonics of the wave A. In order to reproduce the higher numbered even harmonics, a similar transmission line 21 may be used, having a length which is 1/8, or some odd multiple thereof, of the wave length of the even harmonics to be transmitted by this particular lter. It is realized, of course, that, where the repetition rate of the pulses is sufliciently high, the filters 24 and 25 may be eliminated, filters 'such as the line 34 in the units 26 and 21 being suflicient in such cases.

As is shown in Fig. 1, the buffer tubes 3l, connected to the individual lter circuits, have a common output circuit which is coupled to the upper Vertical deection plate 3B of the cathode ray tube 37, the lower plate 35 being connected to ground. The cathode ray tube 31 may be of any well-known form and the details thereof form no part of my invention. Briefly, means are provided therein for developing and projecting an electron ray 38 against a fluorescent screen 39 at the end of the envelope. The inf tensity of the ray 38 is controlled by the potential applied to the control electrode 4D. The ray is caused to scan the target in a horizontal plane in a well-known manner through the action of the ray-deflecting plates 4l to 'which suitable scanning potentials, such as the saw-tooth wave 42, `are supplied from a suitable potential source, not shown. Vertical deflection of ray 38 is effected by the 'potential supplied to plates 36 by the filters 24-21. r 4 e The b'uier tubes 30 have been shown as triodes having control electrodes connected to the com'- mon points of `resistors I6 and I1 by coupling capacitors 43. Buffer tubes 3l, likewise, have been shown as triodes whose control electrodes are connected to the output of the individual lters by means of coupling capacitors 44. Operating potential for "the buier tubes 30 and 3l may be supplied from Iany suitable source, such as the battery 45. It vwill be realized, of course, that while buffer tubes 30 and 3| have been shown as triodes, any suitable multiple-electrode type of electron discharge device may be used.

The operation "of the noise reduction circuit 4thus far described may be shown quite clearly by reference to Fig. 3 in which curve B represents the instantaneous 'spectrum of 'the output ofthe detector I3 inthe absence of 4a pulse signal A.

YUnder such conditions, curve B represents 1the noise energy received in 'the amplifier 'Il and transmitted t'o the l'te'r circuit. This noise i`s uniformly distributed over the entire frequency band of the receiver. YCurve C represents 'the noise energy transmitted by the `lter system of my invention. As is 'at 'once apparent, the amount of noise which passes through the ilte'r is greatly reduced over that present in the` output of the detector 13. Thus the lte'r unit 25 passes the noise energy occurring at the 'pulse repetition rate fp, while the u'n'it 26 passes the Anoise energy occurring at the frequency 2L. Ultimately, the units 26 'and 21 are'opera'tiv'e'at the higher harmonics `to pass the noise energy present at the higher lmultiples of fp.

Curve D of Fig. 3 represents the spectrum of the'output of thefilter circuit when vthe amp1ifier Il 'is receiving a pulse 'of the wave A. AEach individualco'mpone'ntof the pulse is transmitted by the nlter circuit tuned to that particular fre- :quency together with thenoise components which lie 'withinheband -of the filter. The components of pulse signal and noise transmitted by all of the filters are added in the output circuits of the buffer tubes 3| and supplied to the plates 36 of the cathode ray tube 31. Since the amount of noise energy appearing in the output of the filter circuits is but a small portion of that present in the input thereof, the tendency of the noise to `mask the desired signals is greatly reduced. Hence, weaker signals may be detected and reproduced on the cathode ray tube.

Referring to Fig. 2, there is shown the block diagram of a radio echo system embodying the noise suppression circuit illustrated in Fig. l. 'I'he antenna |00 radiates the pulses of signals transmitted to it from transmitter These pulses of radiated signal impinge upon a distant reflecting surface and produce echoes which are received by the antenna |00 and supplied to the receiver |02. It is essential for best operation of my radio echo system that the antenna be trained on the reflecting surface so that reiiected signals are received continuously by the antenna and conducted to the receiver |02. Such a result may be obtained in the case of a moving refiecting object, for example, an aircraft or similar body, by the use of suitable follow-up means (not shown) in conjunction with the directional antenna array |00. With this arrangement a continuous succession of echo pulses are received by `antenna |00 and supplied to receiver |02. In the by means of the filter system |02' described in Fig. 1.

In one particular form of the invention, a single band pass lter is used in the filter system |02. This may be the filter 24 which is tuned to the first harmonic of the transmitter pulse. The intensity of the single frequency output wave of filter |02' is limited to a predetermined intensity by the limiter |03 and the output wave of the limiter is supplied to the phase comparator |04.

Operation of the transmitter |0| is controlled by keying pulses of voltage supplied thereto from a multivibrator |05. So controlled, the transmitter |0| supplies to the antenna 00 a wave of signal similar to the wave A, shown in Fig. 1. It may be seen that this output wave is impressed, likewise, on receiver |02. Since this direct signal is many times stronger than the echo signals received by antenna |00 and amplied in receiver |02, even after the elimination of all frequencies but the first harmonic, and limiting in the limiter |03, the dilerence in intensity of the two signals is so great that the phase variation is hard to detect. In order to overcome this diiculty, the receiver |02 is desensitized during periods at which pulses radiated by the transmitter reach the receiver directly and a pulse of voltage having an intensity equal to the intensity of the output wave of the limiter |03 is supplied by the multivibrator |05 to the phase comparator |04.

The desensitizing means for the receiver |02 comprises the master oscillator |06 which generates a train of pulses which may, for example, be shaped to have the general form of the output wave A of transmitter l0 I, but of slightly greater duration, and which may have a frequency which is either equal to or some multiple of the frequency of the output pulse of transmitter |0|. The output of master oscillator |06 is supplied to receiver |02 in a manner well known in the art to place on all the electron discharge tubes of receiver '|02 a gating signal 'of such a sort that these tubes are cut ofi during the periods that this pulse is supplied. In this manner the master oscillator |06 acts as a desensitizing means to supply a blanking pulse to the receiver during the period at which pulses radiated by the transmitter reach the receiver directly.

In the operation of the distance measuring system described above, the multivibrator .|05 supplies a keying pulse of voltage for controlling the operation of the transmitter. So controlled, the transmitter |0| supplies to the antenna 00 a wave of signal similar to the wave A shown in Fig. 1. At the same time, the multivibrator |05 supplies a reference voltage, or signal, of limited intensity to the phase comparator |04. This reference signal is first passed through a filter system |08 similar to the filter |02' so that single harmonic waves supplied to the phase comparator |04 by the receiver |02 and the multivibrator |05 are of the same frequency. During operation of the transmitter |0|, the master oscillator |06 provides a blanking pulse to the receiver |02 which desensitizes the receiver 02, protecting its circuits from the high intensity signals developed in the transmitter |0|. At the end of the keying pulse which controls the operation of transmitter |0|, the transmitter is silenced. Antenna |00 is now free to receive reflected or echo pulses and supply them to receiver 02. In the receiver, all of the component frequencies, with the exception of the first harmonic, are filtered out. The first harmonic, after limiting to a predetermined intensity in the limiter |03, is supplied to the phase comparator |04. The phase comparator |04 may be of any desirable type, and preferably indicates directly on the scale |01 the difference in phase between the selected harmonics of the reference voltage and the echo signal.

As previously stated, the frequency of recurrence of the pulses of the master oscillator |06 may be either equal to, or some multiple of, the recurrence of pulses of the transmitter |0|. The desirable relationship between these frequencies and the manner in which it may be determined become apparent from the following explanation. When a blanking pulse having the same frequency as the keying pulse operates to eliminate the transmitted pulses, the incoming noise is interrupted at the rate of the blanking pulse. This may possibly produce some component of the pulse repetition rate which is passed by the tuned circuit 24 of the receiver. This component unites with the component of the echo pulse and the resultant wave conducted to the phase comparator |04 by the receiver |02 is a false indication of the distance of the reflecting object.

Depending on particular operating conditions, the component of the pulse repetition rate which appears in the output circuit of receiver |02, because of the amount of noise present therein, may not be objectionable; under such conditions, blanking the receiver at the frequency of the keying pulse is permissible. vIf, however, the cornponent of the keying pulse frequency introduced by noise should be objectionable, the operation of the receiver may be interrupted at a rate which is twice the rate of recurrence of the keying pulse, or some higher multiple. Such a blanking pulse is effective to completely block the transmitter pulse from the receiver and, at the same time, does not result in the production of any components at the fundamental keying pulse frequency. I y

It fwill Abe realized that the above discussion concerning `the relation or the frequency of vibration 'df the oscillator rl B6 andthe multivibrator i115 'is pertinent zonly .to instances in which my incise reduction system is applied `to a receiver employing a single band pass lter and `used for measuring the phase difference between transmitted and reflected pulses. As Ain the previous case, it is desirable that the yantenna array be trained on the reflecting'object so that a suincient number of reflected pulses are received to build up the single selected harmonic in the re.- ceiver 162.

Preferably, the output from the master oscillator l*Itli may also be supplied to the multivibrator 105. Ina manner well known to the art, multivibrator may be Vdesigned to generate -a wave of a form similar to that generated by the oscillator `|06 and .consisting of a series of vpulses which recur, depending upon the relationship desired, at vintervals corresponding 'either to the same .frequency or toa sub-multiple `'of the frequency 4of recurrence lof the blanking `pulses Supplied by the master `oscillator |06 to the receiver. In this manner, the blanking pulses for the receiver |62 are synchronized rigidly with the keying pulses for `the transmitter IUI.

It is thus seen that .my invention provides a .receiver .for .a pulse signalling system in which the noise is reduced to a very low level, One particular application where this invention is very useful is in a radio echo system used for distance measuring. It maybe applied with equally good .results to direct point-tO- point communication systems. By yuse of a blanking pulse, not only are the transmitter signals eliminated as a source of error in .the distance-measuring circuit, but .also the receiver circuits are protected `from .the high intensity signals of the transmitter. It is obvious that modifications of the system may 'be made. rIhus, for example, instead of the phase comparator Hill, the cathode ray oscilloscope 31 of Fig. l may be used in .the system of Fig. 2.

While I have shown particular embodiments of my invention, therefore, it will be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and Y scope of my invention.

What I claim as new and desire to secure by l Letters Patent of the United States is:

1. In a signalling system in which pulses of high frequency oscillation are radiated at regular intervals and a continuous succession of evenly spaced vpulses are received, the combination of means for transmitting said pulses, keying means connected to said transmitting means, means for .receiving and detecting said pulses, and means Y'between said receiving 'means and said reflecting object. said indicating means comprising means for supplying a reference signal having the same time occurrence as said transmitted pulse, means in lsaid receiving means for selecting a single harmonic component of said reflected pulse, and means for measuring the phase angle between -said reference signal and said component.

3. In combination, means for transmitting a pulse of high frequency oscillation at regular intervals, means for receiving a continuous succession of said pulses after reflection from a reflecting object, means for desensitizing said receiving means during transmission of said pulse, and means for indicating the distance between said receiving means and said reflecting object, said indicating means comprising means for supplying a reference signal having the same vtime occurrence as said transmitted pulse, means in said receiving means for selecting a single harmonic component of said reflected pulse, and means for measuring the phase angle between said reference signal and said component.

4. In combination, means for transmitting a `pulse of high frequency oscillation at regular intervals, means for receiving and detecting rsaid pulse after reflection from a reflecting object, means including a blanking pulse for desensitizing said receiving means during periods of operation of said transmitting means, and means for reducing the noise present inthe output of said receiving means, said noise reducing means comprising a plurality of resonant circuits tuned respectively to the lfrequency of said pulse and at least 4one harmonic thereof, and means for decoupling each of said circuits from the remainder of said circuits.

5. In a receiver for a pulse signalling system in which a regularly recurring pulse of high frequency oscillation is supplied to said receiver, an input circuit and an output circuit, and means for reducing the noise translated between said circuits, said means comprising a plurality of narrow band pass lters tuned respectively to the frequency of recurrence of said pulse and harmonies thereof, and means for decoupling each of said filters from the remainder of said filters.

v6. In a receiver for apulse signalling system in which a regularly recurring pulse of high frequency oscillation is supplied to said receiver, an input circuit and an output circuit, means for reducing the noise translated between said circuits, said means comprising a plurality of narrow band pass filters tuned respectively to the frequency of recurrence of said pulse and harmonics thereof, said filters being connected in parallel, and coupling means of high impedance connected between each of `said lters and said input and output circuits.

7. In a pulse signalling system in which a regularly recurring pulse of high frequency oscillation is transmitted and a continuous succession of echo pulses iis received after reflection from a distant object, .means for receiving said echo pulses, means :for selecting `a single harmonic component from `said echo pulses, means for limiting the intensity of said component to -a predetermined intensity, means for supplying a reference signal having an intensity comparable to that lof vsaid component and having the same time occurrence as ysaid `transmitted pulse, and means for measuring :the phase angle between said reference signal and said component.

Y8. In a signalling system, the combination of means for radiating a pulse of high frequency oscillation at regular intervals, means for vreceiving a continuous succession of said pulses after 2,416,895 9 10 reflection from a reflecting object, means for de- REFERENCES CITED sensitizing said receiving means for short evenly spaced intervals having a time occurrence which includes said radiation interval and a frequency The following references are of record in the le of this patent:

which is a multiple of the frequency of said pulse, 5 UNITED STATES PATENTS means for selecting a single harmonic component Number Name Date of said reflected pulse, means for supplying a ref- 1 446 890 Espensched Feb 27 1923 erence signal having the same time occurrence 2055883 Terry Sem; 29 1936 as said radiation interval, and means for meas- 2083344 Newhouse'et al. Jun'e 8 1937 uring the phase angle between said reference sig- 10 2553339 Wilson nnept 16: 1941 nal and said component.

EVERHARD H. B. BARTELINK. 

